1. Field of the Invention
This invention relates to a system for treating a malfunctioning heart. The invention provides for the cardioverting/defibrillation of a malfunctioning heart, as well as the possibility of overcoming a tachycardia and bradycardia manifestations without resorting to either cardioverting or defibrillating the heart. The invention also may involve sensing and treating asystole, ischemia, early infarction and heart failure.
2. Description of the Prior Art
In recent years, substantial progress has been made in pacemakers and in the development of cardioverting/defibrillating techniques for effectively treating various heart disorders and arrhythmias. Past efforts have resulted in the development of implantable electronic pacemakers and standby cardioverters-defibrillators which, in response to the detection of an abnormal cardiac rhythm, discharge sufficient energy via electrodes connected to the heart to depolarize and restore it to normal cardiac rhythm. An early example of this cardioverting/defibrillating technique is disclosed in U.S. Pat. No. 3,942,536 of Mirowski et al., the technique involving responses to a sensed peak right ventricular systolic pressure dropping below a fixed predetermined level and not returning above the predetermined level for a given period of time.
Efforts have also been directed toward developing techniques for reliably monitoring heart activity in order to determine whether cardioversion/defibrillation are desirable or necessary. Such techniques include monitoring ventricular rate or determining the presence of fibrillation on the basis of a probability density function (PDF). A system using the PDF technique statistically compares the location of points of a cardiac waveform with the expected locations of points of the normal waveform. When the waveform becomes irregular, as measured by its probability density function, an abnormal cardiac function is suggested. The latter technique is described in U.S. Pat. Nos. 4,184,493 and 4,202,340 both Langer et al.
A more recent system, as disclosed in U.S. Pat. No. 4,475,551 of Langer et al. utilizes both the PDF technique to determine the presence of an abnormal cardiac rhythm and a heart rate sensing circuit for distinguishing between ventricular fibrillation and high rate tachycardia (the latter being indicated by a heart rate above a predetermined minimum threshold), on the one hand, and normal sinus rhythm or a low rate tachycardia (indicated by a heart rate falling below a predetermined minimum threshold), on the other hand.
Still further, research in this area has resulted in the development of a heart rate detector system which accurately measures heart rate from a variety of different electrocardiogram (ECG) signal shapes. One such system is disclosed in U.S. Pat. No. 4,393,877 of Imran et al.
Despite these past efforts and the level of achievement prevalent among prior art systems, there are potential difficulties and drawbacks which may be experienced with such devices.
The U.S. Pat. No. 4,770,177 of Schroeppel discloses a pacer which paces a heart in accordance with the heart/pacer rate needed to produce a required cardiac output while a person is exercising or undergoes emotional stress in response to changes in venous blood vessel diameter. The pacer is adapted to be implanted in a human body and has a pulse generator and control circuitry, which may be realized by a microprocessor. A pacing lead adapted to be implanted in a heart has a tip electrode adapted to engage and supply pacing pulses to a right ventricle of a heart. A piezoelectric sensor determines changes in a diameter of a vein in the human body. Computing circuitry, including the control circuitry, relates the changes in venous blood vessel diameter with the required pacing rate needed to supply a desired cardiac output, and causes the pacer to pace the heart at the required rate when the heart is not naturally paced. The pacer of Schroeppel is not combined with any cardioverter/defibrillator.
Currently antitachycardia systems detect arrhythmias primarily by sensing rate and perform inadequately in the differentiation of hemodynamically stable from unstable rhythms. These devices, for example, may fire during a stable supraventricular tachycardia (SVT) inflicting pain and wasting energy; damage to the heart may result.
A commonly used implantable antitachycardia device is the automatic implantable cardioverter-defibrillators which is commercially available under the model designations 1500, 1510 and 1520 from Cardiac Pacemakers, Inc. whose address is: 4100 North Hamlin Avenue, St. Paul, Minn. 55164. These devices continuously monitor myocardial electrical activity, detecting ventricular tachycardia (VT) and ventricular fibrillation (VF), and delivering a shock to the myocardium to terminate the arrhythmia. This cardioverter-defibrillator has been shown to reduce the mortality rate in patients with malignant arrhythmias with initial studies at Johns Hopkins Hospital and Stanford Medical Center demonstrating a 50 percent decrease in the anticipated total incidence of death, as reported by Mirowski et al., "Recent Clinical Experience with the Automatic Implantable Cardioverter-Defibrillator", Medical Instrumentation, Vol. 20, pages 285-291 (1986). Arrhythmias are detected by (1) a rate (R wave) sensor and (2) a probability density function (PDF) which defines the fraction of time spent by the differentiated electrocardiogram between two amplitude limits located near zero potential. Presently, the functional window of the PDF is wide to permit the detection of both VT and VF, and therefore, this device functions essentially as a rate-only sensing system. As reported by Mirowski, "The Automatic Implantable Cardioverter-Defibrillator: An Overview", JACC, Vol. 6, No. 2, pages 461-466, (August, 1985), when an arrhythmia fulfills either the rate or PDF criteria, the device delivers Schuder's truncated exponential pulse of 25 Joules some 17 seconds after the onset of the arrhythmia. The device can recycle as many as three times if the previous discharge is ineffective with the strength of the second, third and fourth pulses being increased to 30 Joules. After the fourth discharge, approximately 35 seconds of nonfibrillating rhythm are required to reset the device. The Mirowski et al., supra, and the Mirowski, supra publications set out, in summary form, background material relating to the defibrillating/cardioverting arts against which the present invention was made to correct the ischemia (in a closed-loop fashion). Closed loop intravenous drug delivery systems have been developed (and are undergoing evaluation) for the treatment of heart failure. Such systems could be incorporated into an inplantable device to permit the delivery of electrical therapy (pacing/cardioversion/defibrillation) as well as drug therapy, to correct a malfunctioning heart.
In addition to the standard automatic implantable cardioverter-defibrillator characterized by the above-noted, dual detection algorithm, a variant of the device which features a sensing system that relies only on the analysis of heart rate is also available. This "rate-only" version of the known cardioverter-defibrillator preferred by some investigators, is more sensitive than the dual detection version unit and theoretically less likely to miss ventricular tachycardias with narrow QRS complexes. It is believed that the "rate-only" system, on the other hand, may be too sensitive, delivering cardioverting/defibrillating pulses too often or too soon, no hemodynamic parameter having been taken into consideration.
One problem with many current systems is that they function primarily as a rate-only sensing systems and may fire for nonmalignant as well as malignant tachycardias. These firings are not benign; potentially endangering myocardium wasting energy and inflicting pain on the conscious patient, all distinct shortcomings and disadvantages.
External ST segment monitoring systems are commercially available. These systems compare the normal or baseline ST segment of an ECG to that during normal exercise or activity to determine whether the change is significant and indicative of ischemia. Such monitoring systems are currently worn on the patient's waist or over the shoulders, and no active treatment is offered (since ischemia is only identified after the recording is complete, and the tape is scanned). It is possible that this information can be acquired in real time, such that appropriate drug therapy could be delivered to correct the ischemia (in a closed-loop fashion). Closed loop intravenous drug delivery systems have been developed (and are undergoing evaluation) for the treatment of heart failure. Such systems could be incorporated into an inplantable device to permit the delivery of electrical therapy pacing/cardioversion/defibrillation) as well as drug therapy, to correct a malfunctioning heart.